This invention relates to the preparation of epoxides.
Vicinal epoxides are valuable chemical intermediates and monomers useful in making epoxy adhesives and various heat- and solvent-resistant polymers. A well-known process for making vicinal epoxides from olefins involves the oxidation of the olefinic double bond with aqueous chlorine to form the chlorohydrin and reaction of the chlorohydrin with a base to make the epoxide. However, a major disadvantage of this process is the production of an equivalent of HCl from the aqueous oxychlorination step and another equivalent of inorganic chloride from the reaction of the base with the chlorohydrin intermediate. In the case of epichlorohydrin, the conventional preparation uses the same chemistry with the added initial step of chlorinating propylene to allyl chloride which produces an additional equivalent of HCl.
Ethylene oxide is prepared by oxidizing ethylene with molecular oxygen over a silver catalyst. However, this method is not applicable to other olefins because of low selectivity and the formation of by-products. Another method using oxygen involves oxidizing a hydrocarbon such as isobutane or isopropylbenzene with air to the corresponding tertiary hydroperoxide and then reacting the hydroperoxide with an olefin in the presence of a transition metal catalyst. A disadvantage of this process is the formation of co-product alcohol which must be sold or recycled.
Hydrogen peroxide and peroxy acids are other reagents which have been used to epoxidize olefins. Chemical and economic disadvantage of such methods have precluded their use on a large scale.
It is known that cyclic carbonates can be decomposed to form epoxides in the presence of various catalysts. Such a process particularly directed to the preparation of propylene oxide by decomposition of propylene carbonate in the presence of a sulfonium or phosphonium halide or any of certain metal salts is described in U.S. Pat. No. 4,069,234.
U.S. Pat. No. 4,261,906 discloses a process for the preparation of vicinal epoxides by heating an unsymmetrical .beta.-haloalkyl carbonate in the presence of a small but effective amount of a quaternary ammonium or phosphonium salt at a temperature of about 25.degree. C. to 250.degree. C. U.S. Pat. No. 4,069,234 discloses the preparation of vicinal epoxides by heating a corresponding alkylene carbonate in the presence of a catalytic amount of a catalyst selected from the group consisting of phosphonium halides, sulfonium halides, sulfoxonium halides, and certain metal salts.
Many of the methods described hereinabove for the preparation of epihalohydrins, involve the reaction of a base with a 2,3-dihalopropanol. Such 2,3-dibromopropanol is prepared by the halogenation of an allyl alcohol, which produces the undesirable by-product, 1,2,3-tribromopropane. The reaction of this undesirable by-product with a base produces 2,3-dibromopropene, which is difficult to separate by distillation from epibromohydrin.
What is needed is a process for the preparation of halohydrins in which no hydrogen halide by-product is prepared. What is further needed is a process in which unwanted by-products, such as 2,3-dibromopropene, are not prepared.